Fix drush logging to be compatible with latest Drupal 8.1.x code

Otherwise drush gives following error when using drupal-8.1.x 

PHP Fatal error:  Trait 'Drupal\Core\Logger\RfcLoggerTrait' not found in /root/.composer/vendor/drush/drush/lib/Drush/Log/DrushLog.php on line 12
Drush command terminated abnormally due to an unrecoverable error.                                                                      [error]
Error: Trait 'Drupal\Core\Logger\RfcLoggerTrait' not found in /root/.composer/vendor/drush/drush/lib/Drush/Log/DrushLog.php, line 12